A new commemorative heralding what is expected to be the largest stamp exhibition ever held in North America, the Ameripex '86 show that will be taking place in Chicago a year from now, has just been put out by the United States.

The square stamp, similar to last year's Eleanor Roosevelt issue, had its first-day ceremony in conjunction with the finale of Compex '85, a major annual stamp show in the Chicago area that was held in Rosemont, a Chicago suburb. The Postal Service says it will be available tomorrow at all post offices and postal stations.

Next year's exhibition is expected to attract more than 100,000 people to the exhibits from more than 50 countries and to the several hundred sales booths of dealers and representatives of many of the 166 member nations of the Universal Postal Union.

A major international stamp show is held in this country every 10 years, and the Chicago show, May 22-June 1, 1986, is it for this decade. Interphil '76 was held in Philadelphia at about the same time a decade ago. It helped celebrate the 200th anniversary of the United States.

The first major international stamp show in the United States took place in 1913. The second, an exception to the 10-year span, was in 1926, also in New York. The only other exception to the decade intervals was the Century International Exposition, which was held in 1947 instead of 1946 because of World War II and to mark the 100th anniversary of the first postage stamp from the United States.

The United States began issuing commemoratives for its international philatelic exhibition for the fifth show in New York in 1956 and continued with the sixth, held in Washington, and the Interphil show a decade ago.

The new commemorative is a stamp-on-stamp issue, reproducing the better part of the 1-cent ultramarine Benjamin Franklin regular issue that was in use a century ago. It has been set against a tan background, with the text matter proclaiming Ameripex in three lines of black type at the top and the postal data in red at lower left. The 22-cent stamp was designed by Richard D. Sheaff of Chestnut Hill, Mass.

There could be no more appropriate figure than Franklin to rally collectors to the cause of American philately. He is No. 1 in the Scott catalogue, which numbers each stamp chronologically beginning with the 1847 issue. He is on a 5-cent stamp and George Washington, on a 10-cent stamp, is No. 2. The 5-cent stamp paid for the regular first-class postage rate under 300 miles; the 10-cent over 300 miles. He shares with Washington the distinction of having appeared on every American definitive issue from 1847 up to the 1965 Prominent Americans series.

He was the first postmaster general of the new nation, and before that the deputy postmaster general for the Colonies under the British. He produced a profit for them for the first time in half a century. Britain set up the colonial mail system in 1710. Franklin was appointed the colonial postmaster in 1753 after 16 years as postmaster of Philadelphia and held the post for 21 years, including patriotic missions abroad.

The new stamp has been produced in five colors by two processes: red, black and blue by intaglio and tan and gray by offset lithography. The selvage of each post office pane of 48 stamps has a single-digit intaglio plate number in black and a two-digit plate number in offset tan and gray colors. Each pane also has the three standard marginal inscriptions.

Collectors of first-day-of-issue cancellations have a deadline of June 24 -- orders must be postmarked by that date -- and the usual alternatives in ordering.

Collectors acquiring and affixing stamps on envelopes, which must bear addresses, should send the first-day covers to Customer-Affixed Envelopes, Ameripex Stamp, Postmaster, Rosemont, Ill. 60018-9991. No remittance is required.

Collectors preferring full processing by the Postal Service should send their addressed envelopes to Ameripex Stamp, Postmaster, Rosemont, Ill. 60018-9992. The cost is 22 cents per stamp affixed on a cover. Personal checks are accepted, cash is unwelcome, payment by stamps is refused.

The following dates are the first days of issue scheduled for June for stamps from the United States, the United Nations and Canada.

United States -- 7: Sylvanus Thayer 9-cent Great Americans Series, Braintree, Mass. 02184; 8: School Bus 3.4-cent coil Transportation Series, Arlington, Va. 22210; 11: Stutz Bearcat 11-cent coil Transportation Series, Baton Rouge, La. 70821; 14: Abigail Adams 22-cent commemorative, Quincy, Mass. 02169; 20: George Wythe 14-cent postal card, Williamsburg, Va. 23185; 21: Buckboard 4.9-cent coil and Ambulance 8.3-cent coil Transportation Series, both Reno, Nev. 89510.

United Nations -- 26: Six stamps for 40th anniversary of U.N. UNPA, Box 5900, Grand Central Station, New York, N.Y. 10163.

Canada -- 21: Banff National Park $2 definitive for 100th anniversary of national park system; 28: Canada Day 10 stamps featuring forts; 30: International Pharmaceutical Federation commemorative. Philatelic Service, Canada Post, National Philatelic Centre, Antigonish, Nova Scotia, Canada B2G 2R8.